<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>dayName</title>
</head>

<body>
    <div>打开浏览器控制台查看结果</div>
    <script>
        const dayName = (date, locale) =>
            date.toLocaleDateString(locale, { weekday: 'long' });
        console.log(dayName(new Date()));
        console.log(dayName(new Date('09/23/2020'), 'de-DE'));
    </script>
</body>

</html>